The story of Santa Elena Canyon is also the story of the Rio Grande itself.
Geologically speaking, this is a rather young river. The establishment of the
Rio Grande drainage system is partially responsible for the change from
deposition to erosion in the park. Also playing a possible role is the decline
in Basin and
Range activity, which involved the creation of
fault-block
mountains (particularly noted on the
east side of the park) due to
activity on high-angle
faults. The Basin and
Range tectonism began
about 25 million years ago in the park and had pretty much ended by two million
years ago. As a result of the erosion resulting from the Rio Grande drainage
system, you now have former depositional areas, such as Tornillo
Graben, just to the east of
the Chisos Mountains and occupied by thick deposits of sand and gravel, cut by
arroyos and canyons such as
Estufa Canyon. However, the most spectacular
result of the advent of the Rio Grande are the canyons along its course:
Mariscal Canyon, Boquillas Canyon, and
the most dramatic canyon of all, Santa Elena.

When the Rio Grande drainage system evolved to a facsimile of what it is
today is not completely settled; however, the bulk of evidence appears to
support an earlier, 2.25 million year BCE, date. The theory is that at this
time the northern ancestor of the river cut through a divide at the south end
of the Hueco Bolson that
lies to the northwest of the park and integrated with another river to the
southeast, completing a channel to the Gulf of Mexico. This would mark the
approximate time the river began to cut down through the Santa Elena
Limestone, forming the
canyons along its route.

One may ask how it is possible for a river to cut such a deep (1500 feet)
and narrow (down to 25 feet at one location) canyon. First, the river must
become entrenched in its bed. This could possibly happen with rapid erosion in
otherwise resistant rock. Limestone in an arid climate would seem to be ideal
for entrenchment due to the fact it is soluble in water. This might mean the wet
limestone bed of the river would be cut into rapidly (relatively speaking) while
the dry banks would maintain themselves. Limestone in the river bed would be
attacked by a combination of dissolution by chemical means and abrasion by
sand and gravel carried by the swift currents. Once this process got going, a
canyon like Santa Elena could be formed in one or two million years. It would
only take an average erosion rate of about half a millimeter or less per year
to do it.

Below is a photo taken on Mesa de Anguila that shows the canyon incision
from above. Young-earth creationists would say the sediments that would become
the limestone of the canyon were loose and easily eroded as the flood waters of
Noah receded, carving out the canyon. There are two really big problems
with this idea. One is that the rock is limestone, which is deposited slowly.
Floods, Noah's included, don't deposit limestone. The other problem is that if
the sheer cliffs of the canyon were of unconsolidated material, they would
collapse. They would not be competent enough to support themselves. This support
calls for extremely strong rock. In fact, were the canyon to deepen
to the point where the rock on the bottom could no longer support that above
it, the bottom of the cliff would explode outward. This sort of thing can, and
has, happened in deep mines, in which the roof, sides, or even the floor of the
mine explodes into the mine opening. Miners call this a "bump". Not exactly
like your ordinary speed bumps! The receding water could not have cut down very
far into unconsolidated sediment before the walls would
slide or slump.

In the following picture you can see the shoulder of rock that you must cross
over in order to get to the path that runs by the river. The rock units labeled
are the Santa Elena Limestone, Sue Peaks Formation, Del Carmen Limestone,
Telephone Canyon Formation, and Glen Rose Limestone, all of lower
Cretaceous age. Limestone
accumulates slowly in seas, or even lakes, mostly due to the deposition of
shells and tests of marine life, but also due to chemical deposition. Since
calcium carbonate, of which lime consists, is more soluble in cold than warm
water, much limestone accumulates in warm, tropical seas. That was the case for
the Santa Elena and Del Carmen. The Telephone Canyon Formation is also
limey, consisting of alternating beds of limestone and
marl, which is a sort of dirty
limestone. The Glen Rose Limestone, below the Telephone Canyon, has a sort of
"stairstep" look to it, as you can see in the second photo below.

It is unclear to me whether or not the Telephone Canyon overlies the Maxon
Sandstone at this location. The new USGS map (Scientific Investigations Map
3142) does not divide them up, but
maps the combination as "Ktm". I understand the Maxon is not found as a
sandstone everywhere in the park, as it changes to more of a limestone unit
from north to south. Geologists believe the terrestrial material that
appears in the Telephone Canyon and Maxon is due to uplift and erosion north of
Big Bend. The Del Carmen then marks the resumption of limestone marine shelf
deposits. So, where is the Sue Peaks Formation? It is out of sight between the
Del Carmen and the Santa Elena in the photo and consists of beds of limestone
and shale. When all of these
sediments were deposited, Big Bend was part of a shallow, warm sea (and, in
fact, was located close to the equator due to
continental
drift.)

The canyon might be longer than it is were it not for the Terlingua Fault,
which is (photo below) responsible for the dramatic northeastern scarp of Sierra
Ponce on the Mexican side and Mesa de Anguila on the Texas side. At the mouth
of the canyon the rock on the left (northeast) in the photo was dropped about
3000 feet compared to that on the right. The throw gets even larger in Mexico,
to around 4300 feet. At the canyon mouth, the fault lowers the upper Cretaceous
Aguja Formation, which mostly lies beneath relatively recent sediment along the
river (but there are some outcrops), down to the topographic level of the Glen
Rose Limestone. Note how the resistant beds of the Santa Elena and Del Carmen
form cliffs whereas the less resistant Sue Peaks forms slopes.

The Basin-and-Range era Terlingua Fault is right at the base of the fault
scarp on the Texas side, indicating that the cliff here has not receded much
since the fault was formed. (It might be appropriate to mention at this point
that the 3000-foot offset did not occur all at once!) Nevertheless, it is
clear that the scarp is slowly receding. (See the virtual field trip
Maxwell Scenic Highway.) At the same time
the canyon is very slowly widening. In the following couple of pictures you see
first a stress fracture in the wall (Del Carmen Limestone) of the scarp. If
that column of rock comes down all at once, it will be quite a spectacle
(hopefully from a safe distance). The second picture is of a small rock
fall.

In addition to the major Terlingua Fault, there are numerous smaller faults
that parallel it and can be seen in the canyon walls. In the photo that follows
you can make out a small
normal
fault cutting through the Glen Rose and Del Carmen limestones but
disappearing under the debris covering the Telephone Canyon Formation. In a
normal fault the hanging wall of the fault (on the left here, to the northeast
as indicated by the arrows) moves down with respect to the foot wall. I
understand these terms came from men mining ores found in some fault zones.
They stood on the "foot wall" and hung their lanterns on the "hanging wall".

Farther up the canyon you can see the following pair of faults, a
normal fault on the left and a reverse fault on the right. A normal fault,
as described above, occurs where the block above the fault plane slips down with
respect to the block under the fault plane. A reverse fault is just the - ahem -
reverse of this. A normal fault is created by tension in the crust (or,
actually, less compression in one direction than in the other) and is
associated with crustal distension. However, a reverse fault is thought to be
created by compression and is associated with crustal shortening. How come you
see both types of fault together here?

Since the fault planes are parallel, it is unlikely they were created by two
separate events. Most likely the Basin and Range crustal extension that created
the (normal) Terlingua Fault also created all the faults seen below. It's just
that block to the right of the reverse fault slipped down relative to the block
to the left, making it appear to be a compressional feature. Thus, the reverse
fault is due to an episode of random motion in an otherwise extensional
process.

The canyon trail is indeed awe inspiring. Sheer walls of 1500 feet rise
above the river, placid when this photo was taken. With the use of the Rio
Grande for drinking water and agriculture, you have to wonder what the future
of the canyon will be (geological future, that is). It would appear that the
rate of erosion would be reduced due to the reduced flow. On the other hand,
there is little topographic relief between the river bed in the canyon and the
surface of the land on the other side of the Terlingua Fault. Since fault
movement apparently ceased at about the same time the Rio Grande began cutting
into the Santa Elena Limestone, was there a waterfall where the river reached
the scarp? Or was the topographic ground level on northeast side of the fault
always close to that of the bed of the canyon? In that case the erosion of the
ground level may have kept up with that of the bed of the canyon. The mouth of
the canyon may then have been similar to how it appears today, except the fault
scarp started out rather low but kept growing higher and higher as erosion went
to work on both on the canyon bed and the less resistant material northeast of
the canyon. There are other scenarios you can come up with. What would
you have seen here, say, 500,000 years ago?

Speaking of erosional forces slowly widening the canyon, the picture below
shows a large block that has fallen from somewhere on the canyon wall. Not too
far beyond this block, the river trail ends. Note the desert varnish on the
canyon walls.

This block could be of either Del Carmen or Santa Elena Limestone. The
interesing thing about this block is that it records what has occurred along a
bedding plane. In the following photo you see what appears to be what is left
of a layer of clay. The "pockets" in the clay look to me (I'm no expert, mind
you!) like the scour marks I've seen in other sedimentary units in Arkansas. If
so, that would mean you are looking at the top of a bed in this photo.
Experienced geologists can use features like this to determine if a bed is
right-side up or overturned. When folding is severe, you can get a significant
amount of overturned beds. Of course, no folding here, just an exposure due to
a rock fall, so there is little geological interest in whether this is the top
or bottom of a bed.

Perhaps somewhat more interesting is another block beside this one. Note the
vein in the limestone. It is a vein of
chert, a variety of quartz.
This may consist of the shells of organisms, such as radiolarians and diatoms,
that build their shells out of silica. Why it is a vein, I don't know (and
neither, apparently, do geologists - there are at least a couple of ideas).
Maybe there was a time when silica-shell builders were predominant and a silica
ooze formed on the floor of the sea, or maybe it is the result of a
diagenetic process, which
is a change in the sediment after it was deposited, for example by an
accumulation of chert over time from fluids rich in silica. The chert looks
"concretionary" to me,
so I favor the latter view. The presence of chert may mean this rock is of Del
Carmen Limestone, as that unit is known to have chert layers and nodules

An observant person will note that the rocks dip up-canyon. That is, as you
proceed up the river on the path, you are next to younger and younger rocks. In
the following picture see how the Telephone Canyon Formation disappears upstream
due to its dip. You are looking to the west-southwest when looking up-canyon,
and the rocks have perhaps a 15° dip to the southwest (not quite parallel
to the canyon walls) and a
strike that runs
northwest to southeast. (To find the strike of a bed, place a ruler
on it such that it is horizontal. The compass direction of the ruler is the
strike.) This strike and dip orientation holds on both Mesa de Anguila and
Sierra Ponce and is due to a large
monoclinal fold resulting
from the Laramide
mountain-building episode. Assuming the monocline dates to the same time as the
related Terlingua uplift and Terlingua monocline to the north, it was formed
sometime between 70 and 50 million years ago.

Now, for a sort of artsy turn in the field trip, here is the view at the end
of the trail. Swimming or boating hereafter. At this point you are next to the
Del Carmen Limestone; the Telephone Canyon is below ground level. The trail
itself is on material,
mostly sand, accumulated as the river waters slowed. Since the sand appears to
accumulate at about the point the top of the Telephone Canyon Formation reaches
the level of the river, there may be some connection here. There does seem to be
a narrowing of the river (caveat: I made no measurements) at this point, so it
is possible this is the reason for the sediment deposition. However, there is
also a bend in the river about here, so that may be important also. At the river
bend in the distance, you can see the Del Carmen, Sue Peaks, and Santa Elena
rock stack.

On your way out of the canyon, looking east, you can make out terraces of
the Rio Grande, labeled as they are in the USGS map. Also in the picture is
an
intrusion of
mafic magma. The word "mafic"
is a combination of "magnesium" and "ferric/ferrous" (iron), indicating the
two generally predominant metals in the magma. Most likely this is a
basaltic intrusion like many
others found in the park. The numbers, 1-3, refer to the age of the river
deposits comprising the terraces: youngest, older, and oldest, respectively.
The deposits consist largely of gravel, sand, and silt, well-rounded by the
action of water. The tops of these terraces correspond to former flood plains
of the river. Erosion has left these depositional remnants isolated, and, of
course, will eventually remove them altogether.

One last picture shows pretty much the same view as above but several years
earlier. Note the differences that have occurred. For one thing the trees and
brush on the Mexican side have greatly diminished, but the major geological
change is in the river - ever forming and reforming itself. What seems to us
humans as permanence, is but a tick of the geologic clock.